A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103, which in turn vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. The cochlea 104 includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The scala tympani forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid filled cochlea 104 functions as a transducer to generate electric pulses that are transmitted to the cochlear nerve 113, and ultimately to the brain. Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104.
In some cases, hearing impairment can be addressed by an auditory prosthesis system such as a cochlear implant that electrically stimulates auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along an implant electrode. FIG. 1 shows some components of a typical cochlear implant system where an external microphone provides an audio signal input to an external signal processing stage 111 which implements one of various known signal processing schemes. The processed signal is converted by the external signal processing stage 111 into a digital data format, such as a sequence of data frames, for transmission into a receiver processor in an implant housing 108. Besides extracting the audio information, the receiver processor in the implant housing 108 may perform additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through an electrode lead 109 to an implanted electrode array 110 which penetrates into the cochlea 104 through a surgical opening in the outer surface of the cochlea 104. Typically, this electrode array 110 includes multiple electrode contacts 112 on its surface that deliver the stimulation signals to adjacent neural tissue of the cochlea 104 which the brain of the patient interprets as sound. The individual electrode contacts 112 may be activated sequentially, or simultaneously in one or more contact groups.
Following implantation surgery, the mastoid bone continues to grow considerably in children. One recent article reported that the size of the mastoid bone expands 0.6-0.9 cm in length and 0.4 cm in width in the first year, followed by half again the growth until the age of 6-7 years, and thereafter slower growth until reaching adult size. The mastoid cells are about 3-5 cm2 at one year old, followed by a linear growth till the age of 6 (1-1.2 cm2/year) leading to an adult size of 12 cm2. This growth in the mastoid bone and its air cells is important to consider when implanting children with cochlear implant systems. As the mastoid bone of the implanted patient grows over time, the electrode lead should be long enough to compensate for that growth so that the electrode array is not pulled out of the cochlea.
FIG. 2A shows structural details of a cochlear implant electrode arrangement at the electrode opening 201 into the implanted cochlea 104. Immediately after the insertion procedure, the electrode array 110 tends to lie toward the outer lateral wall of the spiral-shaped cochlea 104. Over time, growth of the mastoid bone can tend to pull back on the electrode lead 109 to retract the electrode array 110 back out through the electrode opening 201, as shown in FIG. 2B. Such post-surgical electrode retraction pulls the nearest electrode contact 112 away from its intended target neural tissue within the cochlea 104 back toward the electrode opening 201, or even further, back outside the cochlea 104 into the middle ear 104. This can produce pain sensation in the patient when that electrode contact 112 is energized. Usually in such circumstances, that electrode contact 112 will be inactivated and fewer electrode contacts 112 remain for use to produce sound sensation.
The degree of pull back varies depending on how deeply the electrode array 110 is inserted into the cochlea 104, how well the electrode lead 109 is looped in the mastoid opening, how well the electrode opening 201 is packed with fascia material, and the specific geometry at the electrode opening 201. Currently when the electrode lead 109 is longer than required (typically the case for small children), then it is looped in the mastoid opening so that it can compensate the growth of the mastoid bone. The looping of the electrode lead 109 differs from surgeon to surgeon and also varies with the patient's specific anatomy. There is no one standard looping procedure. In addition, excess slack in the electrode lead 109 that unloops out of the mastoid bowl due to growth of the mastoid bone can press against the overlying skin, which can be uncomfortable and cosmetically unsightly.
Various approaches have been attempted to resist such post-surgical lead retraction. A cork-shaped stopper has been used to tightly wedge the electrode lead in the electrode opening. An anti-retraction skirt has been implemented on the electrode array at the electrode opening which is made of polymer material that swells when contacted by the liquid preilymph medium, thereby holding the electrode array in place. The electrode lead has been mechanically clipped to the bony bridge to decouple the electrode array from mechnical forces on the electrode lead. Other electrode arrangements contain an internal malleable material on either side of the electrode opening which maintains a bent shape after full insertion of the electrode array to resist retraction. A surgical group in Hannover Germany has added to the implant electrode a wing of flexible silicone material that can be fixed to a groove in the bony material on the outer surface of the cochlea near the electrode opening. All of these efforts have suffered from various issues that leave each an imperfect solution.